Fiber optic cables provide high-bandwidth pathways for communication systems and the like. Generally speaking, the fiber optic cable designs are based upon the intended environment for use. By way of example, fiber optic cables used for outdoor environments typically have a water-blocking mechanism for inhibiting the migration of water along the cable and suitable low-temperature performance. Whereas, indoor cables typically have flame-retardant characteristics for meeting indoor requirements. Additionally, some fiber optic cables are designed for meeting both water-blocking and flame-retardant characteristics.
Outdoor fiber optic cables typically include a polymer cable jacket such as polypropylene or polyethylene that acts as an environmental barrier for providing protection for the cable core within the cable jacket. However, the polymer cable jackets used for outdoor applications typically are not suitable for meeting indoor flame-retardant requirements such as general-purpose, riser, plenum, or the like. In other words, using polypropylene or polyethylene without modification for the cable jacket typically prevents it from passing the flame-rating test. Moreover, one or more of the cable core components such as buffer tubes, thixotropic greases or gels, optical fiber ribbons, and the like may have a relatively high fuel-loading characteristic that add to the difficulty in meeting flame-retardant ratings.
One approach for meeting flame-retardant ratings is using special blends of cable jacket materials, thereby making them flame-retardant. For instance, fiber optic cables having flame-retardant ratings might use an outdoor cable jacket material having one or more flame-inhibiting additives. Additionally, flame-retardant fiber optic cables may also use other cable components such as special tapes or the like that act as a flame barrier or char forming layer, thereby helping meet the desired flame-retardant requirements.
Illustratively, a flame-retardant polyethylene (FRPE) or flame-retardant polypropylene (FRPP) can include one or more inorganic additives for flame-inhibiting, smoke suppression, or the like. For example, suitable flame-inhibiting additives include aluminum trihydrate, metal hydroxides, or the like that are blended into the polyethylene or polypropylene for producing a flame-retardant polymer. However, using relatively high-levels of flame-inhibiting additives within the cable jacket polymer or other cable components adds considerable expense to the material and hence the fiber optic cable, thereby making their cost prohibitive.
Another approach for meeting flame-retardant ratings is using materials that are inherently flame-retardant. By way of example, highly-filled polymers such as polyvinyl chloride (PVC) are inherently flame-retardant and used because of their characteristics when burned (i.e., they have intumescent characteristics and excellent char performance). Pure PVCs are inherently flame-retardant because of their high chlorine content. However, PVCs also require additives for use in fiber optic cables that can undermine their flame-retardant characteristics. For instance, PVCs must typically include one or more plasticizers since pure PVCs are relatively stiff materials and the fiber optic cables must be able to bend relatively easily for routing, storage, and the like. These plasticizers are flammable so a balance between the competing characteristics of flame-retardancy and flexibility must be achieved while still being economical for production.
Another level of complexity is added when fiber optic cables must also provide water-blocking features in addition to flame-retardant characteristics. For instance, if a thixotropic grease or gel is used for water-blocking it acts to increase the fuel loading for the fiber optic cable, thereby making it more difficult to meet burn requirements. Additionally, the fiber optic cable must also be able to withstand long-term exposure to water. Thus, there has been a long-felt need for fiber optic cable designs that are suitable for meeting all of the required performance characteristics while still being economical for production.